Charge from 12V auxiliary outlet

Battery must be tested after 10 hours rest/no use.

 

You can safely use the cigarette lighter port as it shuts down when the car is not Ready.
In Ready, it will have no impact on the 12v battery. (I think the Optima is near end of life, but this won't affect that)

 

Maybe, maybe not.
Having another battery connected might change the charging characteristics of the converter.....might.
Based on just the one measurement, I too think that the Optima probably is near death. 12.3 with no load is WAY too low.
Lots of reasons to get it tested/replaced now......and the CPAP might be the least of those.

 

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I don't think there would be a problem, however I would be tempted to add a simple diode in the cable used to charge the battery to prevent any current (intentional or otherwise) flowing from the battery being charged, back into the electronics of your car) (…just my "WorryWart" personality, I guess! )

 

Correct!
In addition: Besides Ready and Off there is Acc (push start without brake). Avoid this mode for charging! In this mode you would be using only the auxiliary battery.
- This could complete discharge it, because there is no support from the HV system.
- If the voltage of the auxiliary battery is below the voltage of your external battery or similar, you can't charge it.

 

My nearly new battery often has a voltage of 11.6 to 11.8 V and everything is fine!
Mostly I'm driving only 2 x 8 min per day. That's not much time for the converter to support the complete 12 V system and charge the auxiliary battery. (A classic alternator would be quicker.)
And the keyless system and my OBD-Bluetooth dongle drain the battery. Because of this low voltage I replaced the auxiliary battery. But it's the same with the new one. But it's no problem!

 

No it is not "fine".

With no load, a reading of 11.6 is almost fully DIScharged.
Even if the car still runs, the battery won't last long if left flat like that for long periods.

You need to measure the battery voltage while the car is "running". It should be 14 V or a bit more.
If not, your charging system is not working.

Then, if the problem really is too many short trips you really need to get a full charge into the battery at least once a week.......with a longer trip or an automatic battery maintainer (tender).

If left the way you describe it, the "normal" 6 year life of an AGM battery might be cut to 2 years.......or less.

 

In Ready the voltage is 13.8 V.
The replaced battery was the original one. It was about six years old and didn't caused any problems.
I'm using an intelligent wall charger one or two times a year and in addition one times in regeneration mode for the last four years.
The battery is only needed for booting the system and not for starting the engine. In emergency cases this can do my small Li Ion Power Pack. I've tested it.

 

Barely adequate. That voltage won't charge the battery much. It might indicate that the battery is drawing too much charging current.

Since all your readings seem a bit low, I think you might want to verify the accuracy of your voltmeter.

If the readings are accurate, there is something there not quite right.
You might get by with it like that for a long time but that doesn't mean that it is "fine".

At the very least, you should connect your maintenance charger once a week for a few hours.

 

Using the same Scangauge, my Gen 2 said 13.8 and my v says 14.4 volts.